1. Field of the Invention
The invention relates to the prevention of corrosion of ferrous metals in contact with spray water used to cool steel produced by continuous casting processes.
2. Description of the Prior Art
Continuous casting is the process of continuously pouring molten metal from a ladle into complex casting equipment which distributes the liquid, shapes it, cools it and cuts it to the desired length. The casting is continuous as long as the ladle has available metal.
In continuous casting, steel leaving a ladle at about 2800.degree. F. is poured into a trough called a tundish. The bottom of the tundish has one or more openings through which the molten steel is distributed to form slabs or billets in the forming area called the mold. The mold is a water-cooled copper jacket providing for high heat exchange rates. At the start of a cast, a dummy bar is moved close to the top of the mold to completely seal the interior. Mold lubricants high in fluoride salts are added to the molten steel in the tundish to prevent oxidation as well as providing molten lubricity. As the cast starts, this bar is slowly lowered through the mold. The molten metal in contact with the cool mold surface begins to solidify and form a skin. As the newly formed steel shape exits the mold area a series of direct contact water sprays continue the cooling/solidification process. The continuously moving billet or slab then moves through roller guides to the straightening section and then to the runout table for cutting to a specified length.
Spray water that contacts the billet or slab becomes contaminated with iron oxide particles. Contaminated water is processed for reuse by putting it through a scale pit to remove dense, settleable contaminants, and then through filters and heat exchange equipment before returning to the sprays. Failure to remove solids in the water could result in spray plugging which would adversely affect product quality and could even shut down the casting process.
Severe corrosion can occur in continuous casters in the zone immediately below the mold (zero zone). Corrosion results from the formation of hydrofluoric acid from the dissolution of mold powders into the spray water. Calcium fluoride deposition can also occur if the concentration of fluoride reaches the saturation point.
When fluoride salts from the mold powders dissolve in the spray water in the zero zone, hydrofluoric acid forms causing the pH to drop to 2.5 to 3.0. This is a considerably lower pH than is seen in the bulk spray water, which typically ranges from 6.5 to 7.5. To render the spray water less corrosive it has been the practice of some mills to raise the pH of the bulk water with concentrated solutions of sodium hydroxide. While sodium hydroxide has allowed the pH to be elevated control is difficult and, in some instances, pH swings have occurred which allow the pH to reach 10-14. At these elevated pHs the water becomes highly scale forming. Significant deposits of calcium salts can occur causing spray nozzle plugging.
If it were possible to raise the alkalinity in these systems and yet at the same time minimize scale formation, and in particular calcium fluoride scales, an advance in the art would be achieved.